This invention relates generally to a sensor system for remote detection and imaging of objects in a backscattering medium such as air or water. More particularly, this invention relates to a method and apparatus for detecting, locating and/or imaging underwater objects such as mines and submarines from an airborne platform using a novel imaging lidar (light detection and ranging) system which improves imaging though the use of internal or external transmitter referencing.
It is desirable in a number of military and civilian applications to search a volume within a backscattering medium for the presence of certain targets. For instance, moored or bottom mines deployed in ocean shipping lanes are a hazard to navigating ships used both for military and for commercial purposes. For other civilian applications such as law enforcement on the ocean, it is desirable to detect the presence of submerged fishing nets or drug-carrying containers used in smuggling contraband. In or near harbors and beaches, it is also desirable to detect submerged obstructions, cables, pipelines, barrels, oil drums, etc. In strictly military applications, anti-submarine warfare demands an effective means of detecting and locating submarines.
Presently, cumbersome and time consuming wire line devices must be used for detecting underwater targets from remote airborne locations. These devices are lowered into the water and of course, are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging.
An improved and novel system for remote detection and imaging of objects underwater (or objects obscured by other backscattering media which is at least partially transmitting to light such as ice, snow, fog, dust and smoke) from an airborne platform has been described in U.S. Pat. Nos. 4,862,257 and 5,013,917, both of which are assigned to the assignee hereof and incorporated herein by reference. The imaging lidar system of U.S. Pat. No. 4,862,257 utilizes a laser to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. U.S. Pat. No. 5,013,917 relates to an imaging lidar system intended for night vision.
Imaging lidar systems of the type described hereinabove are also disclosed in commonly assigned U.S. Pat. Nos. 4,964,721, and 4,967,270, both of which are incorporated herein by reference. U.S. Pat. No. 4,964,721 relates to an imaging lidar system which controls camera gating based on input from the aircraft onboard altimeter and uses a computer to thereby adjust total time delay so as to automatically track changing platform altitude. U.S. Pat. No. 4,967,270 relates to a lidar system employing a plurality of gated cameras which are individually triggered after preselected time delays to obtain multiple subimages laterally across a target image. These multiple subimages are then put together in a mosaic in a computer to provide a complete image of a target plane preferably using only a single light pulse.
U.S. Ser. No. 565,631 filed Aug. 10, 1990 which is also assigned to the assignee hereof and fully incorporated herein by reference, relates to an airborne imaging lidar system which employs a plurality of pulsed laser transmitters, a plurality of gated and intensified array camera receivers, an optical scanner for increased field of regard, and a computer for system control, automatic target detection and display generation. U.S. Ser. No. 565,631 provides a means for rapidly searching a large volume of the backscattering medium (e.g., water) for specified targets and improves upon prior art devices in performance as a result of having more energy in each laser pulse (due to simultaneous operation of multiple lasers) and a more sensitive detection system using multiple cameras. The several cameras may be utilized to image different range gates on a single laser pulse or several cameras can be gated on at the same time to provide independent pictures which can then be averaged to reduce the noise level and improve sensitivity. Both of these improvements result in higher signal-to-noise ratio and thus higher probability of detection or greater range of depth capability.
While the imaging lidar systems described above are well suited for their intended purposes, there continues to be a need for imaging lidar systems of this type which have improved operational accuracy and efficiency in the imaging of targets enveloped by a backscattering medium, particularly underwater targets. One significant problem with known imaging lidar systems is that the performance of these devices are dependent, in part, on the quality of the laser transmitter's output (e.g., the spatial intensity distribution of the laser output). Any anomalous intensity distributions in the laser spatial intensity distribution at the laser output has a deleterious effect on the gated images viewed from the backscattered light reflected off the target. As a result, prior art imaging lidar systems have employed complex and complicated schemes in an attempt to insure that the laser output has uniform spatial intensity distribution (e.g., lacks any anomalies). Such steps have included, for example, (1) precise alignment of optical components in the laser; (2) maintaining cleanliness; (3) use of channel integrators; (4) radiation projecting devices; (5) use of computer software processing (e.g., filtering) techniques.
In practically all implementations of known imaging lidar systems, the alignment (and maintenance of the alignment) of the various components within the laser transmitter has become the primary contributing factor in demonstration of suboptimal performance in test and evaluation scenarios. These problems have manifested themselves in the form of anomalous signals received from the target area which impede or destroy the ability of the operator to detect targets. As mentioned above, because of the anomalies associated with the laser transmitter, presently known lidar systems require the incorporation of sophisticated automatic target detection algorithms (for identifying actual targets as opposed to anomalies or noise) which in turn, increase requirements for ever expanding computer power. At the same time, efforts have been expended to incorporate radiation projecting hardware to improve the quality of the transmitted beam. Since these devices add to system weight and add a loss component to the output, they are less desirable than other approaches from the standpoint of maximizing signal on target. Unfortunately, none of these steps provide a comprehensive solution to the problem of ensuring uniform spatial intensity distribution; and anomalous transmitted signals are typically still present in the laser output.